The present invention relates to a method for measuring the thickness of a thin layer formed on a substrate by, e.g., plating, coating or painting.
Methods are conventionally known in which the thickness of a layer formed on a substrate is measured by means of ultrasonic waves. According to these methods, ultrasonic waves are generally applied to the layer and substrate through a liquid for ultrasonic propagation, and reflected or transmitted ultrasonic waves are then detected by suitable means.
In one such prior art method using ultrasonic measurement, impulsive ultrasonic waves are applied to the layer, and the layer thickness is measured in accordance with the time lag of reflected ultrasonic pulses. In the case of this method, however, the measurable time lag should be restricted to substantially the same degree as the pulse width of ultrasonic waves. Therefore, the layer thickness d measurable by this method is given by EQU d.apprxeq.C.multidot.T.apprxeq.C/f
where T is the pulse width, f is the center frequency of ultrasonic wave, and C is the acoustic velocity for the layer. When measuring the thickness of a metal layer by means an impulsive ultrasonic wave with the center frequency of 100 MHz, for example, the measurable layer thickness ranges from about 30 .mu.m upward.
Other conventional methods of measurement include a method which utilizes a phenomenon whereby an ultrasonic wave applied to a layer to be measured resonates when the thickness of the layer is equivalent to an integral multiple of one quarter-wavelength of the ultrasonic wave, and a method in which the layer thickness is measured by taking advantage of such characteristics of an elastic surface wave that its acoustic velocity varies with the layer thickness.
The utilization of resonance for the measurement involves the following problems. This method cannot measure a layer having thickness smaller than the quarter-wavelength of the ultrasonic wave in the layer. Additionally, the variation of the amplitude of the reflected ultrasonic wave, caused by resonance, is small. Also, the Q value of resonance is small.
In the method of measurement utilizing the acoustic velocity variation of an elastic surface wave, the so-called V(z) curve of an acoustic microscope may be used to indicate interference between a reflected wave from the layer surface and the elastic surface wave. In this case, however, the substrate must be scanned by an ultrasonic acoustic lens in a direction perpendicular to the substrate, that is, in the z direction, requiring much time for one cycle of measurement. Thus, this method is unfit for on-line measurement. Also, plane scanning by the acoustic lens is unsuited for the measurement of two-dimensional distribution of layer thickness.